The invention relates to a safety system for universal attachment to existing garage door openers. The prior art has suggested a number of safety systems for preventing a door from closing on a person or other obstruction. In many cases, these are for elevators or other horizontally sliding doors providing access to a building. A number of these safety systems have detected the presence of a person in a doorway, not door movement, such as U.S. Pat. Nos. 3,903,996; 4,029,176; 4,261,440; 4,317,992; 4,452,009; 4,458,146; 4,621,452; and 4,706,227. Other door control systems have utilized an apertured plate secured to the door to control the decelerating movement during closing of the door, as in U.S. Pat. No. 4,694,607. Other safety door systems have relied upon a prescribed time interval in which the door is supposed to reach the closed position, and if it does not, then the door is stopped or reversed, as in U.S. Pat. Nos. 4,376,971 and 4,533,901. It has also been suggested that a predetermined load on the motor must be exceeded before the safety device functions, as in U.S. Pat. No. 4,561,691. U.S. Pat. No. 4,701,684 suggests a door or gate operator responsive to an overload of current or voltage drop on the motor to actuate a safety system. U.S. Pat. No. 4,563,625 suggests an encoder on the motor shaft to detect the position of a horizontally sliding door, and to decelerate the door near the closing position.